Ex Situ Analysis of Multijunction Solar Cells Based on Hydrogenated Amorphous Silicon

Abstract

Multiple angle of incidence spectroscopic ellipsometry (SE) has been applied ex situ to support a roll-to-roll photovoltaics (PV) technology applying multijunction thin film hydrogenated amorphous silicon (a-Si:H) based device structures deposited on flexible steel foil. This application demonstrates how PV specialists can adopt currently-available SE methodologies to assist in PV production line development, operation, and troubleshooting. Because this thin film production line employs optically opaque steel foil exclusively as the substrate, a reflection optical experiment based on SE is the natural approach for characterization of the output of the line in terms of layer thicknesses and optical properties. Such information is of direct interest in PV as it enables prediction of the photocurrent generated by solar cells and modules. In this chapter, SE approaches are described for extracting the optical properties of a steel foil substrate and the component films deposited on this substrate that comprise hydrogenated amorphous silicon-germanium alloy (a-Si1−xGex:H) tandem PV in the n-i-p device configuration. These approaches apply analytical models for the complex dielectric functions that describe the doped and intrinsic amorphous semiconductors and transparent conducting oxide (TCO) layers. Application of these models in analyses of measurements performed on complete tandem device structures enables characterization of the thicknesses of the top TCO, which also serves as an anti-reflection coating, and the top p-type window layer, which controls the external quantum efficiency of the device in the blue, as well as the bandgaps of the top cell p-layer and i-layer, and in some cases the bottom cell i-layer. Limitations occur in such analyses when absorption by overlying layers reduces the spectral range of light that reflects from underlying layers, coupled with the similarity in the complex dielectric functions of adjoining underlying layers over the reduced spectral range. For example, it is difficult to separate the i-layer and underlying n-layer thicknesses for this reason. It is also difficult to characterize the bottom cell i-layer when its bandgap is close to that of the overlying top cell i-layer, e.g. in a same-bandgap tandem. In spite of these limitations, analysis of multiple angle SE data establishes critical information on structure and properties that provides guidance to thin film PV specialists on the operation of PV manufacturing equipment and on the expected performance of the manufactured product.